FORCE. 423 



resisting material. If, for example, we double the 

 velocity of a cannon-ball, we quadruple its mechanical 

 effect. Hence the importance of augmenting the velo- 

 city of a projectile, and hence the philosophy of Sir 

 William Armstrong in using a large charge of powder 

 in his recent striking experiments. 



The measure then of mechanical effect is the mass 

 of the body multiplied by the square of its velocity. 



Now in firing a ball against a target the projectile, 

 after collision, is often found hot. Mr. Fairbairn 

 informs me that in the experiments at Shoeburyness it 

 is a common thing to see a flash, even in broad day- 

 light, when the ball strikes the target. And if our lead 

 weight be examined after it has fallen from a height 

 it is also found heated. Now here experiment and 

 reasoning lead us to the remarkable law that, like 

 the mechanical effect, the amount of heat generated is 

 proportional to the product of the mass into the square 

 of the velocity. Double your mass, other things being 

 equal, and you double your amount of heat ; double 

 your velocity, other things remaining equal, and you 

 quadruple your amount of heat. Here then we have 

 common mechanical motion destroyed and heat pro- 

 duced. When a violin bow is drawn across a string, 

 the sound produced is due to motion imparted to the 

 air, and to produce that motion muscular force has 

 been expended. We may here correctly say, that the 

 mechanical force of the arm is converted into music. 

 In a similar way we say that the arrested motion of 

 our descending weight, or of the cannon-ball, is con- 

 verted into heat. The mode of motion changes, but 

 motion still continues ; the motion of the mass is con- 

 verted into a motion of the atoms of the mass ; and 

 these small motions, communicated to the nerves, pro- 

 duce the sensation we call heat. 



